Fatty acid beta oxidation (FBLT)

β-oxidation takes place only under aerobic conditions - it is closely related to the respiratory chain. The individual reactions of β-oxidation of fatty acids are catalyzed by four enzymes :

1. Acyl~CoA−dehydrogenase – the prosthetic group is FAD;
2. Enoyl~CoA−hydratase ;
3. L-3-hydroxyacyl~CoA-dehydrogenase – the coenzyme is NAD + ;
4. β-ketothiolase

The reactions can be summarized in the sequence of dehydrogenation – hydration – dehydrogenation – thiolytic cleavage . The first three reactions are analogous to those occurring in the Krebs cycle starting with succinate (see Krebs cycle):

Oxidation of succinate to fumarate using succinate dehydrogenase - FAD is the cofactor.

1. Oxidation of succinate to fumarate using succinate dehydrogenase - FAD is the cofactor.
2. Addition of water to the double bond in fumarate results in malate catalyzed by fumarate hydratase.
   
3. Oxidation of malate to oxaloacetate using the enzyme malate dehydrogenase – the cofactor is NAD + .

1. Acyl~CoA-dehydrogenase - first oxidation

This enzyme catalyzes the formation of a double bond between the 2nd (α) and 3rd (β) carbons of the fatty acid chain. This is a stereospecific reaction that produces trans-enoyl-CoA. The electron acceptor is FAD . There are different types of dehydrogenases in cells, which differ in the length of the MK chain that they oxidize:

• short MK (4–6 C),
• medium MK (6–10 C),
• long MK (12–18 C).

2. Enoyl-CoA-hydratase

This enzyme catalyzes the hydration of the trans double bond formed in the first step. A hydroxyl group is formed – L-3-hydroxyacyl-CoA.

3. Hydroxyacyl-CoA-dehydrogenase

This enzyme catalyzes the oxidation of the hydroxyl group at the third (β) carbon to a keto group. Electrons are accepted by the coenzyme NAD + .

4. β-ketothiolase

The final step of one turn of β-oxidation is thiolytic cleavage catalyzed by β-ketothiolase. It involves an attack of the SH− group of the coenzyme on the β-keto carbon of the fatty acid chain. The reaction leads to the formation of AcCoA and acyl-CoA, which is two carbons shorter.

One turn of β-oxidation

β-oxidation is a cyclic process, one turn of which can be written as:

Acyl−CoA + FAD + NAD⁺ + HS−CoA → acyl−CoA (2 C shorter) + FADH₂ + NADH+H⁺ + AcCoA

The intermediate product (acyl−CoA 2 C shorter) enters the next round of β-oxidation. Most fatty acids have an even number of Cs, so the last turn converts butyryl-CoA into two molecules of AcCoA.

Yield of complete oxidation of palmitate

To give an idea of ​​the overall yield of fatty acid oxidation, here is the equation and energy balance of the complete oxidation of palmitate:

Palmitoyl~CoA + 7 FAD + 7 NAD⁺ + 7 HSCoA + 7 H₂O → 8 AcCoA + 7 FADH₂ + 7 NADH+H⁺

As you can read in the article on the respiratory and ATP production, article on the respiratory chain and ATP production , we cannot determine the exact amount of ATP produced in the respiratory chain during the oxidation of nutrients. Therefore, please consider the following numbers only as approximate and generally correct quantities. We present them here so that you can compare them with the oxidation of other nutrients, eg glucose. In the respiratory chain, 2.5 (3) ATP is obtained from one NADH and 1.5 (2) ATP from one FADH 2 , which in total represents:

• 7 × FADH₂ = 10, 5 (14) ATP,
• 7 × NADH = 17, 5 (21) ATP
• Oxidation of 8 AcCoA in the Krebs cycle = 80 (96) ATP.

The total profit stopped at a total of 108 (131) ATP. But we used 2 ATP to activate the fatty acid , so the net gain is 106 (129) ATP .

References:

Fontana J., Trnka J., Maďa P., Ivák P. et al.: Transformation of substances and energy in the cell. In: Functions of cells and the human body : Multimedia scripts.